Question

Answer 1

1. 

\(=>t1=\dfrac{\dfrac{1}{2}S}{v1}=\dfrac{S}{42}\left(h\right)\)

\(=>t2=\dfrac{\dfrac{1}{2}S}{v2}=\dfrac{S}{28}\left(h\right)\)

\(=>vtb=\dfrac{S}{t1+t2}=\dfrac{S}{\dfrac{S}{42}+\dfrac{S}{28}}=\dfrac{S}{\dfrac{70S}{1176}}=\dfrac{1176}{70}=16,8km/h\)

Answer 2

2,

a,\(=>S1=\dfrac{1}{2}t.v1=\dfrac{21}{2}t\left(km\right)\)

\(=>S2=\dfrac{1}{2}t.v2=7t\left(km\right)\)

\(=>S1+S2=vtb2.t=>\dfrac{21}{2}t+7t=vtb2.t=>vtb2=\dfrac{10,5t+7t}{t}=17,5km/h\)

\(=>vtb2>vtb\)=> VĐV 2 về đích trước

b,\(=>\dfrac{AB}{vtb}-\dfrac{AB}{vtb2}=0,01=>AB=4,2km\)